scatter plot with fitted straight line

What are a and b? Arrays? If so, what sort of data is in them - there is a limit as to the length of the SERIES formula the chart uses, and when you assign an array, you add the literal values to the formula. That's not a problem if you are using a few integers, but it doesn't take many numbers with lots of decimal places before the formula becomes too long. If you can use a range to hold the data, I would do that.